High power laser systems typically include the laser itself followed by an "optical train" which is a series of mirrors and/or gratings to transport the beam from the laser and to direct it towards a target or workpiece. Some laser systems include a deformable mirror used to control the parameters of the output beam in order to correct for distortion that may be introduced in the laser or in the atmosphere between the optical train and the target. When such a control system is used, a return beam that is composed of radiation reflected from the target is used as input to an optical sensor. A processing system controls the deformable mirror in order to cause the return beam to meet some predetermined criterion. Minimum distortion is provided by a system that includes a shared aperture sampler, which is an element of the optical train that both reflects the high power beam and also reflects and samples the target return beam. It is necessary for this sampler, of course, to withstand the thermal load imposed by the high power beam and also to introduce essentially no distortion in the target return beam. If distortions are introduced, the correction system will be given a false input.
In the prior art, shared aperture samplers have used diffraction gratings on the surface of a mirror and/or buried below the surface in order to deflect a portion of the target return in a different frequency range from that of the laser output beam. The sampled beam is directed at a detector having a suitable frequency response. Another approach in the prior art is the use of a dichroic coating on the front surface of the shared aperture sampler that reflects the high intensity beam and passes a certain frequency in the target return beam. The portion of the target return beam that passes through the dichroic coating then strikes a second reflective surface that deflects it out of the optical train. The gratings suffer from high throughput loss and also are subject to considerable frequency dispersion. The wedges have less dispersion but suffer serious thermal distortion.